Yeast extracts are widely used, e.g. for flavour in the food industries, in microorganism fermentation media, and as health foods. The production of yeast extract is described in literature, see e.g. Kelly, M. (1982) Yeast Extract (In: Industrial Enzymology, Godfrey, T. ed.) or Chae, H. J. et al. (2001), Bioresource Technology 76, 253-258. It is typically manufactured by breaking down the yeast by acid hydrolysis or mechanical or chemical disruption of the cells followed by autolysis with endogenous enzymes to degrade the macromolecular structures of the yeast, in particular the proteins, into the maximum amount of soluble material. Possibly, exogenous enzymes, including proteases such as papain, are added to augment the effect of the yeast's own enzymes. After the enzymatic hydrolysis, the yeast extract is separated from the cell debris and possibly pasteurized and concentrated. Turbidity is a quality measure of the yeast extract. Low turbidity makes concentration and separation easier. Therefore, there is a desire for methods to produce yeast extracts with low turbidity.
Proteases found to be applicable according to the present invention have been previously described. E.g., the protease derived from Nocardiopsis sp. NRRL 18262 is disclosed in WO88/03947 (here the strain is referred to as Nocardiopsis sp. strain 10R) and WO01/58276. Other related proteases which are useful according to the invention are disclosed in WO88/03947, WO04/111220, WO04/111222, WO04/111223, WO05/123911, and WO04/072279.